Daniel John Blackwood

Influence of the space-charge region on electrochemical impedance measurements on passive oxide films on titanium

Both potentiostatically and galvanostatically controlled electrochemical impedance spectroscopy were used to show that the impedance obtained from passivated titanium electrodes is dominated by the capacitance of the space-charge layer. As a result neither the thickness of the oxide layer nor its rate of thinning can be readily determined from impedance measurements. Furthermore, it appears that the corrosion rates of passive metals determined by EIS may significantly overestimate the true rate of dissolution. No significant differences were observed between the impedance data obtained from the galvanostatic and the potentiostatic measurements.

Three-dimensional microfabrication in bulk silicon using high-energy protons

We report an alternative technique which utilizes fast-proton irradiation prior to electrochemical etching for three-dimensional microfabrication in bulk p-type silicon. The proton-induced damage increases the resistivity of the irradiated regions and acts as an etch stop for porous silicon formation. A raised structure of the scanned area is left behind after removal of the unirradiated regions with potassium hydroxide. By exposing the silicon to different proton energies, the implanted depth and hence structure height can be precisely varied. We demonstrate the versatility of this three-dimensional patterning process to create multilevel free-standing bridges in bulk silicon, as well as submicron pillars and high aspect-ratio nanotips.

Freestanding waveguides in silicon

Using a direct-write process for the production of three dimensional microstructures on a semiconductor, freestanding waveguides have been realized in silicon. The waveguides are produced by a focused beam of high energy protons that is scanned over a silicon substrate. The latent image of the scan is subsequently developed by electrochemical etching. Herein the authors report on the fabrication method as well as determining the propagation loss of these structures. Propagation loss values of 13.4 and 14.6 dB/cm were obtained for these preliminary structures for transverse electric and transverse magnetic polarizations, respectively.

Stability of protective oxide films formed on a porous titanium

Abstract Porous titanium has mechanical advantages over its solid counterpart, especially for use in surgical implants, however, its corrosion resistance is not as good. The relative stability of various oxide films on both porous and solid titanium was thus investigates. The point of initial failure of the oxide films on porous titanium was found to be within the depths of the pores. It is also postulated that galvanic coupling to the outer surface, via a mechanism akin to crevice corrosion, may promote the rapid dissolution in the pores. Preliminary, results suggest that thermal oxides offer better corrosion protection to porous titanium than anodic oxides.

Rational Design of Self-Supported Ni3S2 Nanosheets Array for Advanced Asymmetric Supercapacitor with a Superior Energy Density

We report a rationally designed two-step method to fabricate self-supported Ni3S2 nanosheet arrays. We first used 2-methylimidazole (2-MI), an organic molecule commonly served as organic linkers in metal-organic frameworks (MOFs), to synthesize an α-Ni(OH)2 nanosheet array as a precursor, followed by its hydrothermal sulfidization into Ni3S2. The resulting Ni3S2 nanosheet array demonstrated superior supercapacitance properties, with a very high capacitance of about 1,000 F g-1 being delivered at a high current density of 50 A g-1 for 20,000 charge-discharge cycles. This performance is unparalleled by other reported nickel sulfide-based supercapacitors and is also advantageous compared to other nickel-based materials such as NiO and Ni(OH)2. An asymmetric supercapacitor was then established, exhibiting a very stable capacitance of about 200 F g-1 at a high current density of 10 A g-1 for 10,000 cycles and a surprisingly high energy density of 202 W h kg-1. This value is comparable to that of the lithium-ion batteries, i.e., 180 W h kg-1. The potential of the material for practical applications was evaluated by building a quasi-solid-state asymmetric supercapacitor which showed good flexibility and power output, and two of these devices connected in series were able to power up 18 green light-emitting diodes.

The effect of etching temperature on the photoluminescence emitted from, and the morphology of, p-type porous silicon

The temperature at which silicon is electrochemically etched has been found to influence the structure and photoluminescence properties of porous silicon. Decreasing the temperature increased both the current efficiency of the dissolution process and the porosity of the resulting porous layer. Furthermore, a blue-shift was observed in the photoluminescence indicating that the decreased temperature allowed smaller nanocrystals to be formed. An analysis of temperature dependence of the pore initiation and propagation models currently available in the literature failed to yield a satisfactory explanation for the decrease in the average size of the nanocrystals indicated by the results presented in the present paper. Therefore it was proposed that at lower temperature smaller nanocrystals are stabilized due to a combination of their reduced solubility and the increased viscosity of the diffusion layer that leads to a higher localized concentration of silicon ions, thereby allowing smaller nanocrystals to be in equilibrium with their surroundings. The fact that previous authors did not observe blue-shifting highlights the importance of the composition of the etching solution in controlling the quality of the porous silicon produced.

Three-dimensional control of optical waveguide fabrication in silicon

In this paper, we report a direct-write technique for three-dimensional control of waveguide fabrication in silicon. Here, a focused beam of 250 keV protons is used to selectively slow down the rate of porous silicon formation during subsequent anodization, producing a silicon core surrounded by porous silicon cladding. The etch rate is found to depend on the irradiated dose, increasing the size of the core from 2.5 microm to 3.5 microm in width, and from 1.5 microm to 2.6 microm in height by increasing the dose by an order of magnitude. This ability to accurately control the waveguide profile with the ion dose at high spatial resolution provides a means of producing three-dimensional silicon waveguide tapers. Propagation losses of 6.7 dB/cm for TE and 6.8 dB/cm for TM polarization were measured in linear waveguides at the wavelength of 1550 nm.

Influence of anodization on the adhesion of calcium phosphate coatings on titanium substrates.

Electrochemical deposition is an attractive technique for the deposition of calcium phosphate, especially hydroxyapatite, on titanium implants. However, the adhesion of these coatings to the titanium substrates needs to be improved for clinical use. It is demonstrated that anodization of a titanium alloy does marginally increase the adhesion of calcium phosphate coatings. Although scratch test measurements on coatings deposited at a constant potential appear to suggest that adhesion improves with increased thickness of the anodized layer, when a constant current is used to deposit the coatings their adhesion becomes independent of the thickness of the anodized layer. This apparent contradiction is explained by the thicker oxides acting as larger series resistors that reduce the magnitude of the current density when deposition is conducted at a constant potential. The resulting lower current density is responsible for increased adhesion of the calcium phosphate coating. It was also observed that surface roughness affects the interfacial adhesion strength between the coating and the titanium substrate, with a more adherent coating being formed over a rough surface. However, adhesion becomes independent of surface finish at levels smoother than 600 grit, suggesting that mechanical interlocking is not the sole force at play.

Fabrication of silicon microstructures using a high-energy ion beam

We report an alternative technique which utilizes fast proton or helium ion irradiation prior to electrochemical etching for three-dimensional micro-fabrication in bulk p-type silicon. The ion-induced damage increases the resistivity of the irradiated regions and slows down porous silicon formation. A raised structure of the scanned area is left behind after removal of the un-irradiated regions with potassium hydroxide. The thickness of the removed material depends on the irradiated dose at each region so that multiple level structures can be produced with a single irradiation step. By exposing the silicon to different ion energies, the implanted depth and hence structure height can be precisely varied. We demonstrate the versatility of this three-dimensional patterning process to create multilevel cross structure and free-standing bridges in bulk silicon, as well as sub-micron pillars and high aspect-ratio nano-tips.

In Situ Electrochemical Functionalization of Porous Silicon

The high surface area of porous silicon (PSi) makes it attractive for use in chemical and biological sensors. Selectivity, however, will require tailoring its interfacial characteristics with organic molecules. This paper describes in situ functionalizing of PSi during its electrochemical formation with 1-heptyne and 6-heptynoic acid. Fourier transform infrared spectroscopy (FTIR) confirmed the attachment of the organic molecules, which were able to take part in subsequent chemical reactions. However, some disadvantages of in situ functionalization were noted, such as a reduction in the thickness and porosity of the PSi layer, along with incomplete coverage of the PSi; Si-H stretches were still observed on the FTIR spectra. Nevertheless, when the in situ functionalized PSi was treated in boiling 1-decene, all the Si-H groups were converted to S-C bonds, without the 1-decene replacing the molecules attached in the in situ process. Hence, combining in situ and ex situ functionalization, or adding a mixture of organic molecules into the etching solution, may enable different organic molecules with varying chemical functional groups to be incorporated on a single PSi specimen. Finally, the possibility of using PSi functionalized with carbonyl groups as an alcohol sensor was demonstrated by using FTIR to observe reversible shifts in the frequency of the carbonyl stretch on exposure and removal of methanol vapor.